Many therapeutic polypeptides suffer from short terminal in vivo half life and poor thermal stability when injected into a subject. Short plasma half life is commonly due to fast renal clearance as well as to enzymatic degradation occurring during systemic circulation. The long half life time is usually required for a therapeutic polypeptide to achieve its optimal efficacy. Increasing the in vivo residence times of the therapeutic polypeptides could decrease their dosing frequencies and make them more convenient for the patients to use.
PEGylation has been widely utilized to extend the half life of a therapeutic polypeptide (see review paper [1-4], patents 1-9). PEGylation changes the physical and chemical properties of the biomedical molecule, such as its conformation, electrostatic binding, and hydrophobicity, and results in an improvement in the pharmacokinetic behavior of the drug. In general, PEGylation improves drug solubility and decreases immunogenicity. PEGylation also increases drug stability and the retention time of the conjugates in blood. However, PEGylation has severe consequences for the biological activities of the protein. The activity of the PEGylated protein usually reduces by 20-50 fold [2, 5](patents 1-9). In addition, the site for PEGylation needs to be carefully decided to avoid interfering with the active site of the therapeutic polypeptide. For some short peptides such as GLP-1, PTH and Calcitonin, it would be difficult to choose the proper site for PEGylation without disturbing the biological activity of the peptides. Moreover, PEG is a heterogeneous mixture of related polymers, its conjugation to a therapeutic polypeptide results in numerous distinct species with similar molecular sizes and chemical properties. This complicates the purification and increases the production costs of the PEGylated products.
It has been reported that fusion of a therapeutic polypeptide with human IgG Fc fragment or human serum albumin (HSA) may significantly increase the half life of the therapeutic polypeptide [6-9] (patents 10, 11, 12). However, recombinant fusion protein with IgG Fc fragment or HSA needs to be produced from eukaryotic systems such as mammalian cell lines or yeast cells, which significantly increases the cost of the recombinant protein.